The present invention relates to a method of forming a pattern of a semiconductor device and, more particularly, to a method of forming a pattern with a uniform line pattern.
In a typical pattern formation process of a semiconductor device, a photoresist pattern is formed over layers to be patterned, such as a silicon layer, an insulating layer and/or a conductive layer, using a photolithography process. The layers are etched using the photoresist pattern as an etch mask, thus forming a desired pattern.
In this photolithography process, the photoresist is first uniformly formed on a wafer using a spin method. The term “photoresist” is a type of photosensitive polymer that undergoes a photo exposure reaction when being exposed to light of a specific wavelength band. The term “exposure reaction” refers to the polymer chains of an exposed portion of the photoresist being broken or further bonded when exposed to light. In general, a photoresist is classified into a positive photoresist in which a polymer bonding chain of an exposed portion is broken and a negative photoresist in which a polymer bonding chain of an unexposed portion is broken. That is, the positive photoresist has its exposure region developed through a subsequent development process, so a pattern of a non-exposure region remains. The negative photoresist has its non-exposure region developed, so a pattern of an exposure region remains.
A solvent of about 80% to 90% existing in the photoresist is evaporated using thermal energy by performing a soft bake, thus drying a photoresist film. Thus, adhesion is improved and stress is reduced by an annealing process.
A latent image of a desired pattern is formed on a surface of the photoresist by performing an exposure process in order to selectively generate a photochemical reaction of the photoresist though exposure energy. In the exposure process, a desired latent image is formed by selectively irradiating the photoresist with a light source by employing the photomask in which a light-shield pattern and a light transparent pattern are formed. The wafer on which the latent image of a desired pattern is formed is inserted into a hot plate oven and then exposed for about 90 seconds. A Post Exposure Bake (PEB) process is then carried out. The photoresist of a portion where bonding has relatively become weak through the exposure process is dissolved in the solvent. A shape of the photoresist formed through this process is called a photoresist pattern. At this time, a portion where bonding is relatively weak (an unexposed portion), of the polymer chain untied by the photosensitive action in the positive photoresist, compared with a portion of the negative photoresist where bonding becomes strong by the photosensitive action, is removed. Meanwhile, there is a chemical amplification type photoresist for forming a complete photoresist pattern by generating acid within the photoresist during an exposure process by employing a difference in the concentration of a photo sensitizer due to a mutual interference phenomenon between incident light and reflected light and then making the acid function as a catalysis by thermal energy obtained in a PEB process in order to selectively remove the photoresist through a chemical amplification action.
However, as semiconductor devices become more highly integrated, a design rule with a smaller Critical Dimension (CD) is used. Thus, there is a need for a technology for forming a contact hole having a small opening size or a micro pattern having a small width. Accordingly, it is an important issue to form a micro photoresist pattern without defects when a photolithography process is performed.
In view of the above circumstance, there has been proposed a method of forming a more micro photoresist pattern by performing a resist flow process on a photoresist pattern. The resist flow process refers to a process of forming a smaller photoresist pattern by applying thermal energy to raise the temperature of the photoresist above its glass transition temperature in order to shrink the pattern though thermal flow of the photoresist pattern.
FIG. 1A is a photograph showing a contact hole pattern on which the resist flow process was performed. FIG. 1B is a photograph showing a line pattern on which the resist flow process was performed. In particular, in FIGS. 1A and 1B, photographs located on the left of arrows indicate photoresist patterns before the resist flow process was performed, and photographs located on the right of the arrows indicate photoresist patterns after the resist flow process was performed.
As shown in FIG. 1A, the resist flow process is suitable to reduce the size of the contact hole pattern formed in the photoresist. However, as shown in FIG. 1B, the size of a line pattern formed in the photoresist is not shrunken even after the resist flow process, but the outline of the pattern has collapsed.